1. Field of the Invention
This invention relates to hydrogen sulfide gas sensors. In particular, it relates to a chemiresistor coating for electrodes used in hydrogen sulfide gas sensors.
2. Description of the Prior Art
Hydrogen sulfide is a toxic gas which has the ability to temporarily deaden the human sense of smell. Therefore, there is an important benefit in being able to detect the presence of hydrogen sulfide gas in the environment.
In addition to health concerns, the presence of sulfide during the production of photographic products may directly affect the quality of the product.
The extent to which one can control the sulfide content in either the atmosphere or in the photographic product manufacturing process depends on the ability to measure it. Therefore, the detection and quantitative analysis of sulfides, even at trace amounts (i.e., ng/ml), must be precise.
A chemiresistor sensing device generally contemplates the use of a power supply transmitting current through a sensor which contains a semiconductor material, such as a metal oxide. The semiconductor material behaves as a chemiresistor. A chemical influence can be caused by an ambient gas interacting with the semiconductor material and can be monitored by a change in the resistance or conductance of the material by the use of electrodes which transmit the change in conductance to a monitor or detector means, such as a voltmeter.
Chemiresistor gas sensors using semiconductor materials comprised of thin film metal oxides, such as tungsten oxide, have shown good sensitivity for detecting reducing gases, such as hydrogen, anhydrous ammonia, hydrazine, propane, butane, methyl alcohol, ethyl alcohol and hydrogen sulfide (H.sub.2 S).
Chemiresistor sensors which incorporate thin films of tungsten oxide as the sensing material have been known to respond selectively and sensitively to hydrogen sulfide gas. The exposure of tungsten oxide to hydrogen sulfide gas results in a decrease in the resistance of the sensing metal oxide. A measurement of the decrease in the resistance of the sensing metal oxide can be used to determine the concentration of the hydrogen sulfide gas. Certain known chemiresistor sensors comprise a resistor layer, such as a heater resistor, an electrical connection to the heater, a support layer, such as an alumina substrate, a conductor layer (often composed of interdigitated electrodes) and a deposited chemical sensing layer most frequently comprised of tungsten oxide (See for example, Jones et al., U.S. Pat. No. 4,822,465).
The manner in which the tungsten oxide semiconductor material is applied to the electrodes is of particular importance because the microstructure resulting from the method or technique of depositing the tungsten oxide layer can affect both the selectivity and sensitivity of the tungsten oxide layer to hydrogen sulfide gas.
The sensors described in Willis et al., U.S. Pat. No. 4,197,089, describe hydrogen sulfide gas sensors with improved selectivity to hydrogen sulfide gas, which comprise a chemically formed sensor film of tungsten trioxide produced by decomposing a droplet of ammonium tungstate contained in solution and deposited on the sensor. The patent also discloses a physically formed sensor film of tungsten trioxide which is produced by sintering tungsten trioxide in the powder form on the electrode surface.
One major disadvantage inherent in the above techniques is an inability to manipulate the microstructure of the film formed. Depositing powdered tungsten oxide and sintering the powder or by placing a drop of an aqueous solution containing ammonium tungstate over the electrodes followed by thermal decomposition are rather crude methods for the creation of a film on the electrode. Uncontrolled microstructure of the film leads to unpredictable sensitivity and selectivity of the sensing film. The inability to manipulate the microstructure of the thin film precludes optimizing the sensitivity and selectivity for a given set of conditions. Since the method used to deposit the thin film will dictate the microstructure of the metal oxide film and, since the microstructure of the metal oxide film may determine the selectivity and sensitivity toward the reducing gas of interest, the method used to deposit the sensing film is very important to its sensing abilities.
Another method for depositing thin films of tungsten oxide on electrodes is referred to in Jones et al., U.S. Pat. No. 4,822,465, which discusses what is known as a radio frequency sputtering technique. This technique contemplates a deposit of the sensing film by sputtering the film onto the electrodes which are, in turn, supported by a substrate. One of the shortcomings of depositing sensing films by sputtering techniques arises when dopants are added to the sensing film. If it is desirable for the sensing film composition to contain a dopant, it is preferred that the dopant be uniformly dispersed throughout the sensing compound to provide consistence in the electrical properties of the film. This is difficult to achieve using sputtering. In addition, sputtering may yield mixtures in which there is either less than or more than the optimal concentration of dopant in the sputtered thin film.
In addition, the radio frequency sputtering technique inherently introduces varying levels of stress into the thin film which may effect the sensing capability of the thin films. This stress results from the inability of the sputtering technique to deposit the sensing film uniformly over the surface of the electrode. Conformance to irregular substrates is often poor with sputtered films.
All of the shortcomings of sputtering enumerated above could be ameliorated by solution casting techniques. However, until development of the present invention, application of tungsten oxide thin films using spin-casting (also known in the industry as spin-coating), dip-casting and spray-casting solution techniques (hereinafter collectively referred to as solution casting techniques), has been unavailable. Until development of the present invention, known technology was unable to provide for the precise and uniform application of tungsten oxide films onto electrodes, because tungsten oxide is insoluble in the aprotic solvents used in solution casting techniques.
Thus, a need still exists for improving the application of thin film metal oxides and, in particular, tungsten oxide to electrodes contained in hydrogen sulfide and other reducing gas sensors.
It is an object of the present invention to provide tungsten compounds that are soluble in aprotic solvents, that can be solution cast and that thermally decompose to provide tungsten oxide or sodium tungsten oxide electrodes for detecting hydrogen sulfide.